The present invention relates to a system for controlling the air-fuel ratio of an internal combustion engine emission control system with a catalytic convertor comprising a three-way catalyst.
Such a control system comprises a feedback control system, the system comprising an oxygen sensor for detecting the concentration of oxygen in the exhaust gases, an air-fuel mixture supply unit, an electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply unit, and an electronic control circuit. The electronic control circuit comprises a comparator for comparing the output signal of the oxygen sensor with a predetermined value, a proportional circuit and an integrating circuit connected to the comparator for integrating the output of the comparator, and a driving circuit connected to the proportional and integrating circuits for producing a driving signal for driving the electromagnetic valve. The oxygen sensor generates an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture induced in the engine cylinder.
The output voltage of the oxygen sensor is higher than a predetermined voltage when the oxygen concentration of the exhaust gases is smaller than a predetermined ratio corresponding to the stoichiometric air-fuel ratio in the air-fuel mixture for the combustion of the mixture and is lower than the predetermined voltage when the oxygen concentration is greater than the predetermined ratio. The control system operates to actuate the electromagnetic valve to correct the air-fuel ratio of the mixture to be supplied to the cylinder to the stoichiometric air-fuel ratio in dependency upon the output voltage of the oxygen sensor. The constant of the proportional circuit and the constant of the integrating circuit are selected to proper values respectively for providing an effective purification reaction in the three-way catalyst during normal operation of the engine during which the air-fuel ratio of the mixture is substantially constant. On the other hand, the air-fuel ratio varies with acceleration and deceleration of the engine and the purification effect of the emission control system decreases by the variation of the air-fuel ratio caused by such engine operating conditions.
It has been found that if the circuit constant is increased depending on the increase of the acceleration and deceleration, decrease in the purification effect of the system may be prevented.